BURSA, Turkey — At least 39 people were killed and dozens more were wounded when a single gunman attacked a crowded Istanbul nightclub about an hour after midnight on New Year’s Day, Turkish officials said.

Interior Minister Suleyman Soylu called it an act of terrorism and said the gunman was still being sought early Sunday.

Sixteen of the people killed were foreigners, the Foreign Ministry said; it was not clear if any were Americans. At least 69 people were being treated at hospitals, Mr. Soylu said.

No one immediately claimed responsibility for the mass shooting, which came as threats against Turkey by the Islamic State and its supporters have increased. It was the fourth terrorist attack in Turkey in less than a month.